Fix api-link for interface force_detach_volume

The api-link for interface force_detach_volume should be
"#force-detach-a-volume", not "#force-delete-a-volume".

Change-Id: I4288aad4084cbfb2b7db873f17d9dab86d66a5c4
diff --git a/tempest/lib/services/volume/v3/volumes_client.py b/tempest/lib/services/volume/v3/volumes_client.py
index 14a3c48..4fb6d2e 100644
--- a/tempest/lib/services/volume/v3/volumes_client.py
+++ b/tempest/lib/services/volume/v3/volumes_client.py
@@ -349,7 +349,7 @@
 
         For a full list of available parameters, please refer to the official
         API reference:
-        https://docs.openstack.org/api-ref/block-storage/v3/index.html#force-delete-a-volume
+        https://docs.openstack.org/api-ref/block-storage/v3/index.html#force-detach-a-volume
         """
         post_body = json.dumps({'os-force_detach': kwargs})
         url = 'volumes/%s/action' % volume_id